clxviii Introduction. 



but by the thickening of a part of the contained protoplasm, so that 

 they are truly unicellular organisms. This cell-wall is readily 

 permeable by water, which first separates it from the contained 

 parenchyma, and ultimately, as in the ease of many other Entozoa, 

 bursts it. The integument may be longitudinally striated or 

 eostate, and may carry delicate or strong bristles, or even vibratile 

 cilia, and more or fewer spines. The contents of the cell-wall are 

 of three kinds : the nucleus, the hyaline protoplasm, and the fatty 

 o-ranules, which give the adult animals their milky ai:)pearance, but 

 which may be wanting in young specimens. The nucleus is placed 

 centrally in the Monocydidea, and in the anterior half of the 

 posterior half of the body in the Dlcyst'idea. It contains a nucleolus, 

 or several small granules. The reproduction of the Gregarinae, so 

 far as it is known, appears to be accompanied by the successive 

 processes of encystation : of resolution of the nucleus and the paren- 

 chyma, firstly, into small round corpuscles a little larger than a 

 human red blood-cell ; and, secondly, into the organisms known as 

 ^pseudo navicellae:' and, lastly, of the change of the amoebiform 

 contents of those bodies into Gregarinae by direct growth. The 

 resolving up, and the rearrangement of the contents of these uni- 

 cellular organisms, resemble the process known in the higher ani- 

 mals as the segmentation of the yolk ; and it is preceded, though 

 not always, by ' conjugation,' which might, had it been an inva- 

 riable antecedent, been compared to sexual congress. 



The Gregarinae have been regarded as merely Amoebae, which are 

 possessed of a better defined external envelope than other Rhizo- 

 poda in adjustment to their parasitic habits. They have again been 

 supposed to have, by virtue of the peculiarities of their reproductive 

 process, and of their first stages of development, to show much 

 affinity to certain fungi. And, finally, whilst the pseudo-navicular 

 cysts bear some resemblance to the ' psorospermiae' of Fishes, the 

 adult Gregarinae, as ordinarily, though not exclusively, found in the 

 digestive or perivisceral cavities of Invertebrata, are by no means 

 unlike the wormlike organisms found in the heart and the voluntary 

 muscles of many Mammals, and known as ' pseudentozoa.' 



